The present invention relates generally to the screens with filter elements that are usually fitted to water intakes, either river water intakes or seawater intakes, to block debris and particles carried by the water drawn in.
A water intake screen is normally on the downstream side of a grid with bars a few centimeters apart protecting it from large debris, either on its own or in combination with one or more other screens of the same type forming part of the same screening station.
Prior art filter elements, which have a mesh size of only a few millimeters, are mobile so that they can be periodically cleared of debris and particles larger than their mesh size, which progressively obstruct all of the mesh and therefore block the filter element.
The present invention relates more particularly to the rotary screens known as “chain filters” that are commonly encountered in water intakes in the United States and in Japan in particular. In these rotary screens, consecutive flexible articulated rectangular filter panels or elements are carried by two chains, one on each side, forming an endless loop of elongate cross section constituting said chain filter.
A filter element of this kind passes cyclically from an immersed position, in which it is progressively charged with debris and diverse particles, to an out of the water position, in which it is subjected to contraflow pressurized water jets to clear it of the particles and debris that have accumulated on its surface in the above manner, to enable it to resume its filtering function when it is next immersed.
At present, the particles and debris entrained by the washing water are usually collected in and drained off via a channel provided for this purpose.
It would seem that until now the fact that the debris and particles are of mineral and vegetable origin and contain lifeforms, in particular fish, has not been a matter for concern except in a few particularly sensitive locations.
This is no longer the case today, when the widespread concern to protect the natural environment is taken much more seriously, especially in the case of protecting aqueous lifeforms, when greater amounts of water are taken from rivers and the sea, in particular by nuclear power stations, and in view of the fact that the problem is aggravated by the current tendency to install for this purpose water intakes with large flowrates on ecologically sensitive estuaries or on the seashore.
Now it is clear that these lifeforms, and in particular fish trapped and entrained by the filter element of a screen of a water intake of this kind, are inevitably removed from the water, often for a long time, are exposed to the usually brutal effect of the washing water jets applied to the filter element while it is out of the water, and suffer the resulting violent splashing in the channel for recovering particles and debris temporarily blocking the filter element in question.
Various systems to be fitted to a water intake screening station to save lifeforms and in particular fish carried by the water drawn in have already been proposed.
However, at present, these are either special filters that in themselves merely save fish, with no general screening function, or conventional screening filters that still remove fish from the water.
One prior art system which avoids removing fish from the water and the resulting trauma is disclosed in French patent No. 7720258. It is used only in rigid drum filters and provides a recovery channel below the lowest water level. The recovery channel is provided with pressure reducing means that induce a reverse flow of water through the filter element that is sufficient to entrain lifeforms retained thereby.
However, this kind of recovery channel is designed only to be installed on rigid filter drums and not on chain filters whereof the filter comprises a succession of flexible articulated filter panels.